Photons are considered ideal for communication of quantum information because they are the most convenient carriers of quantum bits (qubits), yet the lack of photon-photon interactions limits their use for quantum information processing. Achieving nonlinear behavior at the level of single photons—that is, photon-photon interactions and quantum gates controlled by single photons in particular—is a major challenge in realizing quantum networks in which quantum information would be processed by material quantum nodes interconnected by photonic channels.
Because photons do not interact directly with other photons, considerable effort has been put into implementing materially-mediated photon-photon interactions. A significant advancement was the attainment of strong coupling between single atoms and optical micro-resonators in the field of cavity quantum electrodynamics, where the tight confinement of light in tiny volumes leads to extreme enhancement of the electric field associated with photons in the cavity mode.
Most notably, based on a scheme involving auxiliary control fields, recent works have demonstrated: nondestructive measurement of an optical photon; a single-photon phase switch; and a quantum gate between flying photons and a single atom, showing atom-photon, photon-photon, and atom-photon-photon quantum entanglement. All of these effects can be directly applied to photonic routing.
Yet the need in auxiliary control fields (e.g. Raman laser beams or microwave fields, or other classical control fields) seriously limits both the scalability and the speed of such schemes, as each operation of each individual atom or other material node requires a series of a few microseconds-long (typically) control pulses and/or measurements.
In order to enable scalable quantum networks there is a need for a passive device that coherently routes and manipulates photons between multiple ports, enabling conversion of the photons from one wavelength to another, and which is activated solely by single photons, making it compatible with large-scale photonic circuits. This goal is achieved by embodiments of the present invention.